1. Technology Field
The present invention generally relates to liquid cooled x-ray tube environments. More particularly, the present invention relates to bladder system for use in a liquid-filled x-ray tube that enables remote venting for the bladder while containing interior tube liquids in the event of a bladder breach.
2. The Related Technology
X-ray producing devices, such as x-ray tubes, are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray tubes operate in similar fashion. In general, x-rays are produced when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an evacuated enclosure of the x-ray tube. Disposed within the evacuated enclosure is a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk that is mounted to a rotor shaft which, in turn, is rotatably supported by a bearing assembly. The evacuated enclosure is typically contained within an outer housing, which also serves as a reservoir for a cooling liquid, such as dielectric oil, that serves both to cool the x-ray tube and to provide electrical isolation between the tube and the outer housing.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a cloud of electrons to be emitted via a process known as thermionic emission. A high voltage potential is placed between the cathode and anode to cause the cloud of electrons to form a stream and accelerate toward a focal spot disposed on a target surface of the anode. Upon striking the target surface, some of the kinetic energy of the electrons is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The target surface of the anode is oriented so that the x-rays are emitted as a beam through windows defined in the evacuated enclosure and the outer housing. The emitted x-ray beam is then directed toward an x-ray subject, such as a medical patient, so as to produce an x-ray image.
Generally, only a small portion of the energy carried by the electrons striking the target surface of the anode is converted to x-rays. The majority of the energy is rather released as heat. It is critical to remove excess heat produced during x-ray production to prevent failure of the x-ray tube. One common method in dissipating heat involves submerging the evacuated enclosure in a dielectric cooling liquid which, as explained above, is contained within a reservoir defined by the outer housing. The cooling liquid assists in absorbing heat from the evacuated enclosure that is produced therein during x-ray production and dissipating it to the surrounding environment. Such dissipation can be accomplished, for example, via conductive heat transfer between the cooling liquid and the surface of the outer housing. In this way, the operating temperature of the x-ray tube is maintained within acceptable levels.
In many liquid-filled x-ray tubes, one or more expansion bladders are employed in order to maintain a relatively consistent liquid pressure within the reservoir at or near atmospheric pressure (“1 atm”). These expansion bladders are flexible and include a first surface in liquid communication with a portion of the cooling liquid as well as a second surface that is in communication with the tube exterior such that it is subject to atmospheric pressure. During tube operation, heat created as a result of x-ray production is absorbed by the cooling liquid. Absorption of this heat causes the volume of the cooling liquid to expand. In response to this volume expansion, the expansion bladder contracts, thereby expanding the relative size of the reservoir, which relatively reduces the pressure of the cooling liquid.
Similarly, when cooling of the liquid occurs, its volume and corresponding pressure decrease. Expansion of the expansion bladder is then triggered, which reduces the liquid reservoir volume, thereby relatively increasing cooling liquid pressure. The expansion bladder is configured and operated in this manner to maintain the cooling liquid pressure at or near 1 atm (atmospheric pressure) during tube operation, notwithstanding the cyclical temperature changes of the cooling liquid. This in turn enables the fluid-tight seals of the x-ray tube outer housing to be configured for mere liquid containment, and not for liquid containment at elevated pressures relative to atmospheric pressure. This consequently reduces both the complexity and cost of x-ray tube seals, thereby offering added savings for tube manufacturing.
Despite their utility in maintaining constant cooling liquid pressure, several challenges nevertheless exist with respect to expansion bladder use. Many of these challenges relate to the unintended rupture or other failure of the expansion bladder. When such failure occurs, escape of cooling liquid past the expansion bladder can result. Further, because many tube designs require that the expansion bladder be exposed to atmospheric pressure and therefore lack a fluid-tight seal about the entirety of the expansion bladder, cooling liquid that escapes past the bladder can also spill from the x-ray tube entirely. Such spillage is highly undesirable. As can be imagined, liquid escape from the x-ray tube not only presents a contamination problem, but can also be hazardous by presenting a health risk to tube users, patients, or others in close proximity to the x-ray tube.
In particular, x-ray tubes are often employed in connection with medical x-ray scanning devices, such as CT scanners. An x-ray tube utilized in a CT scanner is often mounted on a rotating gantry that achieves high rotational rates during scanning operations. Should the expansion bladder of a CT scanner x-ray tube so positioned fail during use, extensive cooling liquid leakage and dispersal from the tube can occur, resulting in exposure to the local environment, users, patients, etc. As described above, cooling liquid often possesses significant quantities of absorbed heat, which can present a burn risk to those persons who may be exposed to the liquid. Furthermore, some cooling liquids are caustic or otherwise hazardous substances, thereby representing an additional contamination risk. For these and other reasons, expansion bladder failure and its attendant consequences are to be avoided.
In an effort to reduce the effects of expansion bladder failure, some known x-ray tubes hermetically seal the expansion bladder off within the outer housing and isolate it from atmospheric pressure influences. Though this alleviates liquid containment problems should the expansion bladder fail, it nevertheless represents a significant additional expense in manufacturing such tubes, as all fluid-tight seals used in the outer housing must be designed to withstand the elevated pressure that results from such a tube design.
Another attempt at avoiding the above challenges has involved tubes that employ a dual expansion bladder system, wherein a first expansion bladder is backed by a backup second expansion bladder in the outer housing of the x-ray tube. Though this dual expansion bladder design can in certain cases enhance the safety of the x-ray tube in the event of a single expansion bladder failure, both expansion bladders must still be subject to atmospheric pressure, and therefore are still susceptible to the above undesirable consequences should failure of both expansion bladders occur. Further, a dual expansion bladder system is necessarily more complex than a single expansion bladder system, thereby equaling greater production costs and more complication when tube servicing is required, as well as creating more possible failure points, given the extreme operating conditions in which x-ray tubes are often utilized.
Still other challenges are presented with regard to x-ray tube expansion bladders. One of these involves the use of radiation shielding in the x-ray tube. Most x-ray tubes contain some form of shielding to prevent the emission of x-rays from the tube except where intended. Often, such shielding includes a lead lining that is attached to a portion of the outer housing that is susceptible to unintended x-ray emission. This notwithstanding, design constraints often dictate that expansion bladders be positioned within the outer housing at locations that are preferably to be shielded by the lead lining. However, because of the general requirement for the bladder to be exposed to atmospheric pressure, a gap must often be made in the shielding layer in order to enable a portion of the bladder to be exposed to the atmosphere. As a result, additional shielding configurations must be employed in order to compensate for the shielding gap made necessary by the expansion bladder. This translates into increased complexity and cost for the x-ray tube design.
In light of the above, a need exists for an x-ray tube having an expansion bladder system that avoids the above problems. In particular, an x-ray tube having an expansion bladder system that protects from cooling liquid escape in the event of bladder failure is needed. Such a solution should be adaptable to different x-ray tube types and other apparatus without substantially increasing the complexity thereof. Any solution should also alleviate the need for comprising radiation shielding measures of an x-ray tube in order to position the expansion bladder for proper operation. Additionally, any solution should not interfere with the operation of the expansion bladder in maintaining a constant cooling liquid pressure within the x-ray tube.